This invention relates to data processing systems; more particularly to data processing systems of the type having a plurality of subsystems linked by a star coupler.
Star couplers are well known in data processing technology. Systems employing star couplers or couplers similar to star couplers are described, for example, in Metcalf & Boggs, "Ethernet: Distributed Packet Switching for Local Computer Networks," 19 Communications of the ACM 395 (July, 1976); Rawson & Metcalf, "Fibernet: Multimode Optical Fibers for Local Computer Networks," 26 IEEE Transactions on Communications, 983 (July, 1978); Data Processing System Having Dual Channel System Bus, Gunderson, et al. U.S. Pat. No. 4,417,334, Nov. 22, 1983; and Data Processing Subsystem Wherein At Least One Subsystem Has a Local Memory and a Mailbox Memory within the Local Memory for Storing Header Information, U.S. Pat. No. 4,387,441, Kocol, et al., June 7, 1983.
The latter two patents are assigned to the Assignee of this invention. The present invention is usable in the systems disclosed in these patents, which are incorporated herein by reference.
When used in a data processing system having a plurality of subsystems or terminals, a star coupler is typically connected to pairs of transmission lines, with one pair associated with each subsystem. A first transmission line of the pair carries signals away from the subsystem to the star coupler, and a second transmission line of the pair carries signals from the star coupler to the subsystem. When any subsystem transmits or generates a signal or message, that message is received by the star coupler from the first transmission line associated with the subsystem and is directed or passed to every subsystem by way of each second transmission line, including the second transmission line returning to the subsystem that transmitted the message. This of course offers advantages in linking multiple subsystems, since a subsystem transmitting a message receives back the message at the same time each of the other subsystems receives the message. The transmitting system can therefore check for any transmission errors, without requiring a receiving subsystem to regenerate the message.
The function of the star coupler is essentially to take all the signals received on the input side of the coupler on the first transmission lines, logically OR all these first transmission line signals, and redrive the resulting signal to all terminals or subsystems over the second transmission line of each pair.
There is, of course, a potential problem encountered in a system using a star coupler at this central linking section which logically ORs all the incoming signals on the first transmission lines. If a processor or subsystem continues to send message signals on a channel for an extended period of time, the entire system can be tied up. This problem is particularly serious in the situation where a processor fails in the transmit mode.
Another problem which must be addressed by this system is that of a subsystem which intermittently overruns its transmission time. That is, typically in systems of this type, the length of a message that may be sent by any single terminal or subsystem is limited so that the other terminals can contend for access to the common linking means at the star coupler. However, if a terminal overruns, but this situation only occurs intermittently, then the overrun may not be detected, but the efficiency of the star coupling system will be significantly reduced.
Yet another situation which must be accommodated in an overrun detection system is where a terminal or subsystem overruns the defined maximum length of a given message, but then ceases transmission and returns to normal operation. Without a properly designed overrun detection circuit, this terminal could be permanently locked out of the linking means of the star coupler.